The objective of the work is to design better techniques, particularly media, for in vitro fertilization and the culture of preimplantation embryos. The proposal is in two parts. The first part concerns the growth and differentiation of the mammalian blastocyst. Two animal models will be used, the mouse as a minimally expanding blastocyst, and the rabbit as a maximally expanding blastocyst. The response of the blastocysts to different media will be evaluated by measuring three major components of blastocyst development -- cell multiplication, the movement of fluid into the blastocyst and the synthesis of cell surface macromolecules. The initial phase will search for suitable available media that can provide the starting point for the addition of supplements such as amino acids and growth factors. Attempts will be made throughout to apply the principles of good experimental design to optimize the composition of media. The second part concerns finding suitable conditions for fertilizing primate oocytes in vitro. The purpose is to produce primate embryos for genetic manipulation, particularly to increase the numbers of genetically valuable animals for transfer, and by embryo bisection. The technique is also required for producing transgenic primates, which has importance in cancer research. Attempts will be made to increase the efficiency of in vitro fertilization in the rhesus monkey and the common marmoset.